As ATMs (Automatic Teller Machine) and POS terminals are widely used in daily life, financial fraud and crime that make use of card information stolen by skimmers arranged near card readers or POS terminals (i.e., magnetic card reading devices) arises frequently. The skimmer is a card reader equipped with a memory device and it is capable of reading all magnetic strip information of a magnetic card (such as a bank card) and recording it. Generally, the skimmer is installed inside the card reading device or at the front end of the magnetic head of the magnetic card reading devices of ATMs or POS terminals. When a cardholder inserts a magnetic card into the card reading device, the magnetic card information will be acquired by the skimmer before the ATM or POS terminal starts the transaction with the magnetic card. The cardholder will not be aware of the stealing of the magnetic card information at all because the transaction will not suffer any disturbance. Besides, according to a report issued by Mercator, the loss caused by card skimming in USA every year has amounted up to several ten billion dollars and it is still on the rise.
Among techniques for preventing the card reading device from being skimmed, electromagnetic protecting field is generally set up inside the card reading device or at the front end of the magnetic head of the magnetic card reading device so as to disturb signals received by the skimmer, that is, an electromagnetic interference source is arranged inside the card reading device. However, once quality deficiency or fault of the electromagnetic interference source happens, card information is still in danger of being stolen from the magnetic card reading device. Therefore, it is necessary to test the interference source so as to prevent the magnetic card information being skimmed.